Utilizes the 'lme4' package and the optim() function from 'stats'
to estimate (generalized) linear mixed models (GLMM) with factor
structures using a profile likelihood approach, as outlined in
Jeon and Rabe-Hesketh (2012) <doi:10.3102/1076998611417628>.
Factor analysis and item response models can be extended to allow
for an arbitrary number of nested and crossed random effects,
making it useful for multilevel and cross-classified models.